In a manufacturing process of semiconductor devices or flat panel displays (FPDs), plasma is frequently used so as to cause a processing gas to conduct a good reaction at a relatively low temperature in a processing such as, for example, etching, deposition, oxidation, or sputtering. Conventionally, plasma generated by high frequency discharge in a MHz region or plasma generated by microwave discharge in a GHz region is widely used in such plasma processings.
The plasma generated by the microwave discharge has an advantage in that low electron temperature and high density plasma may be generated under a low pressure. In particular, when a microwave introduction window structure incorporating a slot antenna is employed, there are advantages in that large diameter plasma may be efficiently generated and, since no magnetic field is required, a plasma processing apparatus may be simplified.
In such a microwave plasma processing apparatus, however, in order to generate plasma required for a desired process, it is necessary to supply a required processing gas into a vacuum chamber (processing container) in such a manner that the processing gas can be electrically discharged within the chamber. Typically, a microwave introduction dielectric window is attached to a ceiling of the chamber as a ceiling plate. In a plasma generation space within the chamber, since an electric field and radiation power of microwaves are the highest in the proximity of the inner side of the dielectric window (ceiling plate), the highest plasma generation efficiency may be obtained when the processing gas is introduced in the vicinity of that area. For this reason, an upper gas introduction mechanism is widely used in which the upper gas introduction mechanism is configured to introduce a processing gas into the chamber through a gas flow path that extends through the dielectric window.
However, the dielectric window also serves as a microwave propagation path and a number of microwave electric fields are distributed within the dielectric window. Thus, when the processing gas is exposed to the microwave electric fields within the gas flow path of the dielectric window, the processing gas may be electrically discharged. When the processing gas is electrically discharged within the gas flow path of the dielectric window or in the vicinity of an inlet of the gas flow path, unnecessary consumption of a microwave power may be caused. Further, a decomposition product of the processing gas may be deposited to a wall of the gas flow path, thereby causing a reduction in conductance, for example. In the worst case, the dielectric window may be damaged by the electric discharge.
As a method of preventing such abnormal electric discharge within the dielectric window, in the prior art, the wall of the gas flow path within the dielectric window is made of a conductor so as to shield the processing gas flowing in the gas flow path from a microwave electric field. However, in such a method, a gas jet portion of a conductor (metal) facing the plasma generation space may be sputtered due to the attack of ions from the plasma, thereby causing contamination. In addition, since microwave electric fields are electromagnetically shielded, a uniform plasma processing may be disturbed.
As another method, it has been proposed to fill the gas flow path within the dielectric window with an air-permeable electric discharge prevention member (Patent Document 1). The electric discharge prevention member is typically made of a porous dielectric material and is air-permeable since numerous fine pores are communicated with each other therein. Thus, the processing gas may be smoothly sent to the plasma generation space within the chamber. Meanwhile, even if the porous dielectric material is exposed to the microwave electric fields, electrons are hardly accelerated since the space of the gas flow path (the numerous fine pores in the inside of the dielectric material) is too small. As a result, collision ionization of electrons to attain electric discharge does not occur.